The present invention relates to a method for making organohalosilanes by the direct reaction of an organic halide such as methylchloride and powdered silicon in the presence of a copper catalyst. More particularly, the present invention relates to the employment of thermally treated spent direct method contact mass as a make-up catalyst and silicon source for the direct production of organohalosilanes.
Prior to the present invention, organohalosilanes were generally made by the direct reaction of an organic halide and powdered silicon in the presence of a copper catalyst as shown by Rochow U.S. Pat. No. 2,380,995. It was found that as the reaction proceeded the silicon powder became spent which adversely affected yield and selectivity.
Rossmy, U.S. Pat. No. 3,069,452 teaches that improved yields and selectivity of organohalosilanes can be obtained if the copper catalyst used in the Rochow synthesis is replaced with a brittle easily grindable silicon-copper alloy having from 50% to 99% by weight copper and preferably about 12.8% silicon. The silicon copper alloy is then finely ground and mixed with silicon powder and the resulting mixture is sintered in an inert atmosphere prior to reaction with organic halide.
In Shade, U.S. Pat. No. 4,281,149, particles of silicon and copper of less than 40 microns average diameter which were generated during the Rochow process are removed from the reactor, abraded and then returned to the reactor. In Shah et al, U.S. Pat. No. 4,307,242, a portion of direct process contact mass is analyzed for particle size distribution, classified and segregated into a silicon rich fraction which can be returned to the reactor, and a silicon poor fraction which can be removed from the reactor.
In copending application Ser. No. 07/867,657, there is described a method for passivating or stabilizing spent silicon contact mass generated during the direct method for making organohalosilanes by the reaction between silicon powder and an organic halide such as methylchloride. Spent silicon contact mass can have an average particle size in the range of 0.1 to 200 microns. It is difficult to manage because it is pyrophoric in air and cannot be readily moved to an appropriate waste disposal site or be reused. Treatment of spent silicon contact mass by heating it at a temperature in the range of about 900.degree. C. to about 1400.degree. C. under an inert atmosphere has been found to render the spent contact mass substantially unreactive in air and more easily handled. The treating procedure of copending application Ser. No. 07/867,657 provides a significant advance over prior art, methods such as shown by Hosokawa, U.S. Pat. No. 4,724,122, requiring the combining of the spent silicon contact mass with water and granulating the mixture followed by coating the resulting granules with an inert organic powder. Offenlegungschrift DE 313 1732Al and U.S. Pat. No. 4,758,352 to Feldner et al. also describe procedures for reprocessing residues produced during organohalosilane synthesis. Unlike copending application Ser. No. 07/867,657, the residues from DE 313 1732Al and U.S. Pat. No. 4,758,352 are derived from sludge vessels which consist of siliceous solids and liquids such as polysilanes. It is desirable therefore to provide additional procedures for making organohalosilane from spent powders while minimizing waste.